Traditional orthopedic and traumatological fixation systems to facilitate bone fracture healing (osteosynthesis) typically employ metallic hardware, e.g., plates, screws, rods and the like, formed of biocompatible, corrosion resistant metals such as titanium and stainless steel. Typical metallic plates are described, e.g., in the book, F. Sequin and R. Texhammar, AO/ASIF Instrumentation, Springer-Verlag, Berlin, Heidelberg, 1981, at p. 21-22, 55-79, 107-108, 117-122, the entire disclosure of which is incorporated herein by reference. While such systems are generally effective for their intended purposes, they possess a number of inherent shortcomings. For example, metal release to the surrounding tissues has been reported. See, e.g., L.-E. Moberg et al. Int. J. Oral. Maxillofac. Surg. 18 (1989) at pp. 311-314, the entire disclosure of which is incorporated herein by way of this reference. Other reported shortcomings include stress shielding, see P. Paavolainen et al., Clin Orthop. Rel. Res. 136 (1978) 287-293, the entire disclosure of which is incorporated herein by way of this reference, and growth restriction in young individuals, see K. Lin et al, Plast. Reconstr. Surg. 87 (1991) 229-235, the entire disclosure of which is likewise incorporated herein by way of this reference. In infants and young children, there is the risk that metallic plates and screws can sink into and below the cranial bone, as a consequence of skull bone growth, thereby threatening the brain. See, e.g., J. Fearon et al., Plast. Reconstr. Surg. 4 (1995) 634-637, the entire disclosure of which is incorporated herein by way of this reference. Therefore, it is generally recommended that non-functional implants should be eventually removed, at least in growing individuals. See C. Lindqvist, Brit. J. Oral Maxillofac. Surg. 33 (1995) p. 69-70, the entire disclosure of which is incorporated herein by way of this reference.
Especially in maxillofacial and in cranial surgery, metallic mini plates are popular for use. See e.g., W. Muhlbauer et al., Clin. Plast. Surg. 14 (1987) 101-111; A. Sadove and B. Eppleg. Ann. Plast. Surg. 27 (1991) 36-43; and R. Suuronen, Biodegradable Self-reinforced Polylactide Plates and Screws in the Fixation of Osteotomies in the Mandible, Doctoral Thesis, Helsinki University, Helsinki, 1992, p. 16, and references therein, the discloures of which are incorporated herein by reference. Mini plates are small, thin, narrow plates, which have holes for screw fixation. They are typically located on bone, perpendicularly over the fracture to fix the bone mass on both sides of the fracture to each other. Typical geometries of mini plates are described e.g. in U.S. Pat. No. 5,290,281 at FIGS. 6A-6F, the entire disclosure of which is incorporated herein by way of this reference.
The main advantage of metallic plates (like titanium, stainless steel and cobalt chrome molybdenum plates), is that they are strong, tough and ductile so that they can be deformed or shaped (e.g., bended) at room temperature in the operation room, either by hand or with special instruments, to a form corresponding to the surface topography of bone to be fixed. In this way, the plate can be fixed flush on the bone surface to which the plate is applied.
In light of the above shortcomings of metallic plates, however, bioabsorbable plates have been developed for fracture fixation. Longitudinal, six-hole plates were developed for orthopaedic animal studies. See Eitenmuller et al., European Congress on Biomaterials, Abstracts, Instituto Rizzoli, Bologna, 1986, p. 94, the entire disclosure of which is incorporated herein by this reference. However, because of their inadequate strength, some of the plates were broken in animal experiments involving fracture fixation.
A special advantage of bioabsorbable plates is that they can be provided with openings for the insertion therethrough of surgical fasteners (like screws), while allowing means to permit the formation of additional fastener openings therethrough during a surgical procedure at the surgeon's discretion, as has been described in European Patent specification EP 0 449 867 B1, the entire disclosure of which is incorporated herein by way of this reference.
The main disadvantage of prior art bioabsorbable plates is that they can be deformed (bended) permanently and safely only at elevated temperatures above their glass transition temperature (T.sub.g), as has been described e.g. in EP 0 449 867 B1 and in U.S. Pat. No. 5,569,250, the entire disclosures of which are incorporated herein by way of this reference. Below their T.sub.g, the prior art bioabsorbable plates are brittle and break easily when deformed. Only at temperatures above the T.sub.g does the molecular structure of prior art plates have enough mobility to allow shaping (e.g., bending) without the risk of breaking. Accordingly, U.S. Pat. No. 5,569,250 describes a biocompatible osteosynthesis plate that is capable of being used in a secured relationship over a plurality of adjacent bone portions. That biocompatible osteosynthesis plate includes an elongated section having a top face and a bottom face, at least one fastener opening disposed between the top face and the bottom face, and means disposed upon the elongated section to permit the formation of additional fastener openings therethrough, during a surgical procedure. The osteosynthesis plate is in a first thermochemical state in a first configuration and is capable of being converted to a second thermochemical state so that it may be deformed prior to fixation. The first thermochemical state is typically room temperature (operation room conditions) and the second thermochemical state is typically an elevated temperature above the T.sub.g of the polymer material (e.g., for polylactides between 50-60.degree. C.). Accordingly, in order to shape the plates disclosed in U.S. Pat. No. 5,569,250, they must be changed from their first thermochemical state to the second thermochemical state by heating, and thereafter they must be changed again back to the first thermochemical state prior to fixation. Because the thermal conductivity of polymeric materials is poor, the conversion of material to a second temperature is a slow process. Therefore, the clinical use of plates of U.S. Pat. No. 5,569,250 is tedious, slow and complex, especially if the surgeon must shape the plate several times to make it fit exactly to the form of the bone to be fixed.
K. Bessho et al., J. Oral. Maxillofac. Surg. 55 (1997) 941-945, the entire disclosure of which is incorporated herein by reference, described a bioabsorbable poly-L-lactide miniplate and screw system for osteosynthesis in oral and maxillofacial surgery. However, in order to shape the plates of that reference, they first must be heated by immersion in a hot sterilized physiologic salt solution or by the application of hot air until they become plastic, and only then can they be fitted to the surface of the bone.
EP 0 449 867 B1 describes a plate for fixation of a bone fracture, osteotomy, arthrodesis etc., said plate being intended to be fixed on bone at least with one fixation device, like a screw, rod, clamp or corresponding device, wherein the plate comprises at least two essentially superimposed plates to provide a multilayer plate construction. The individual plates of said multilayer plate construction are flexible, so as to permit a change of form of said multilayer plate construction to substantially assume the shape of the bone surface in the operation conditions by means of an external force, such as by hand and/or by bending instrument directed to said multilayer plate construction, whereby each individual plate assumes a position of its own with respect to other individual plates by differential motion along the respecitive surfaces of coinciding plates.
Although the said multilayer plate fits even the curved bone surface without heating of individual plates, the clinical use of multilayer plates is tedious, because the single plates easily slip in relation to each other before fixation. Additionally, the thickness of multilayer plate system easily becomes too thick for cranio maxillofacial applications, causing cosmetic disturbances and increased risks of foreign body reactions.
U.S. Pat. No. 4,671,280, the entire disclosure of which is incorporated herein by reference, describes the manufacturing of a fastener member or staple, by the winding of an oriented bioabsorbable polymeric filament around a forming bar, which winding is carried out at a temperature below the glass transition temperature of the polymer. Ordinarily, winding will be done at ambient temperature. Because the oriented filament is quite stiff, the coils are bowed out slightly from the sides of the forming bar. Thus, the coils do not fully assume the desired fastener member (or staple) configuration until the filaments are heated, which will normally be done during the annealing step (see, e.g., U.S. Pat. No. 4,671,280; Column 5, first two paragraphs). Thus, while U.S. Pat. No. 4,671,280 may describe some bending of drawn filament at an ambient temperature, the bending does not give the desired configuration of the material until the filaments are additionally heated. The filaments are heated during the annealing step to a temperature above the glass transition temperature of the material (see also Example 1 of U.S. Pat. No. 4,617,280).
A need, therefore, exists for a bioabsorbable (bioresorbable or biodegradable) osteosynthesis device, like a plate, which is thin and substantially rigid and substantially deformable at a first thermochemical state, being also dimensionally stable before and after deformation (shaping) in the said first thermochemical state. A need also exists for a bioabsorbable (bioresobable or biodegradable) osteosynthesis plate, which is strong, tough, does not produce a substantial inflammatory response, and which plate can be deformed, yet dimensionally stable at temperatures below the glass transition temperature (T.sub.g) of the material from which the device is made, to facilitate shaping. A need further exists for such a bioabsorbable (bioresorbable or biodegradable) osteosynthesis plate, which is strong, tough, does not produce a substantial inflammatory response, and which plate can be deformed, yet dimensionally stable at room temperature in operation room conditions, to facilitate the shaping of the plate. Likewise, a need exists for such a bioabsorbable (bioresorbable or biodegradable) osteosynthesis plate, which is strong, tough, does not produce a substantial inflammatory response, and which plate can be deformed, yet dimensionally stable in operation room conditions (in the first thermochemical state) to allow its fixation on bone without distortion of the configuration of the bone fragments to be fixed, and which shaped plate is also dimensionally stable at a second thermochemical state, in tissue conditions, when fixed on bone surface to facilitate non-problematic bone fracture healing.